Learn about audio cables
Qualitative analysis of audio cables. Physics and snake oil.
Qualitative analysis of audio cables. Physics and snake oil.
When I started reading about audio cables (interconnects and speaker cables) I found different opinions and some of them contradicted with each other. After a few days of reading I felt tired because there is so much information and contradictions that I didn't know what was right or wrong. Finally I got my own conclusion: every cable sound different. Now, the important questions are: can we perceive the differences? If yes, is it a big difference or just a subtle change? Is it worth to spend a certain amount of money to achieve a specific result? So I decided to narrow my research, trying to understand the physics behind a cable and answering those 3 questions.
A cable is not just a piece of metal that transport electrons from one end to the other one. There are many physical phenomenons happening inside the cores (conductor) and the surrounding areas (dielectric, shield, jacket, other cores, EMI, etc.) and all those phenomenons modify the signal. In summary, a cable is a system that changes the audio signal. I will not go into details because I'm not a physicist, however I would like to highlight some important concepts.
"The best cable is not a cable", but we need them to transmit electrical signals between different modules. A cable will always make signal worst. A good cable will modify the signal just a little bit and a bad cable will modify it a lot. The quality of your cables should be in balance with the quality of your audio system. A rule of thumb is that 10% of your budget should go to cables. A good cable will not enhance the sound of a poor audio system but it can deteriorate the signal of a good audio system. A cable can work fantastic in one system and not that good in other one. OK… those are just a few ideas. Let's have a quick look inside an audio cable.
A common way of modelling a cable is defining the cable as a filter (RLCG circuit). Below there is a quick explanation of each parameter:
Resistance will transform part of the signal into heat reducing the efficiency of the system. This is relatively easy to fix by using good materials and increasing the overall cross section of the conductor. For some people, increasing the area of the cores will create a muddy sound, but this phenomenon can't be explained just because of the resistance.
Because of inductance, the electrical current flowing through one core will generate currents in the same core and cores near by. Signals that were not in the source will be generated and added to the original signal reducing accuracy in sound. Because of capacitance, the cable will store energy as electric charge. This cahrge will be released after a certain amount of time also reducing accuracy in sound.
Ideally all 4 values should be zero, but in real life it's not possible. That's why cable companies work hard to keep those values as small as possible. This is quite difficult because RLCG are correlated: when one value is reduced the other values may increase. Those values depend on many factors such as: geometry, materials, length of the cable, etc. The values may also change depending on the signal strength and frequency because the system is not 100% lineal. Of course, we are talking about very small values that are not necessarily perceptible by the human ear and probably extremely difficult to measure with standard instruments. That's why it's important to focus in the 3 questions of the first paragraph.
The analysis above considers a cable as a low-pass filter which reduces the amount of high frequencies and, even worst, creates a phase (delay in time) between the input signal and the output signal. This is a problem because the phase is different for each frequency reducing accuracy in sound. Normally the cut frequency of the equivalent low-pass filter will be way over 20KHz but it still possible to hear the effects in some cases.
It's easy to find companies using all kind of materials and exotic techniques to manipulate them: OFC and OCC copper, cryogenically treated cores, 999 silver, rhodium plated connectors, gold, etc… It's clear that all those materials will make a difference in the sound but it would be extremely difficult to know which ones are the "best", if that question makes any sense. Crystals inside the material will affect the electrons travelling inside it. The atomic structure will also affect the signal. Oxygen inside the core and corrosion in the surface will do the same. Resistance and self inductance also will affect the sound. Single or multi stranded cores? Insulated multi cores (Litz wire)? There are lots of variables and they are hard to analyse all together.
At the end, everything affects the signal. How much? It's hard to say. I just want to advice to be careful with all the marketing bullshit going around these "sophisticated" and "extravagant" companies trying to sell the "best cable" for ridiculous amount of money. I'm not saying that they sell bad cables, but you should ask yourself, how much are you willing to pay for a cable that may or may not improve the sound of your system? Just be careful and get informed before you spend an eye and a leg for the "perfect cable".
NOTE1: Material of the cores is more related to the cable's self inductance and resistance.
Skin effect is a phenomenon that explains how different frequencies travel across different sections of a metal conductor. This is produced because of self inductance within the core. High frequencies tend to travel in the surface while low frequencies tend to travel evenly across the cable section. Please understand that all frequencies will travel across the whole section of the cable but the current density will vary. High frequencies still travel in the centre of the cable but in a very small proportion compared with the surface.
Now the obvious question, is the skin effect important for audio signals? I don't know. There are special configurations for reducing the skin effect like Litz wires in which many thin cores are insulted transporting the same signal. This is very important in HF communication systems but not sure how much difference it would make in an audio cable.
Summarising, high frequencies will experiment higher resistance which will contribute to the low-pass filter effect explained in section 2.
In a nutshell, cables have insulation around the cores, a jacket that covers all cores to give protection and sometimes a shield is inserted to reduce EMI. Shield can be foil, braided or both.
Non-conductive materials, also called dielectrics, are unable to conduct electricity (they are insulators, below a specific voltage/thickness) but they are able to store electric charge. The capacity of storing electric charge is represented by the "dielectric constant" of the material. The value of the dielectric constant of any material is always greater than 1, being 1 for free space. Air, at 1[ATM] has a dielectric constant of approximately 1.0006. Pure cotton has a dielectric constant around 1.4 and Teflon around 2. Other materials as PVC have higher dielectric constant. Hi-fi cables try to use materials with low dielectric constant. That's why it's common to see Teflon and cotton in top end cables.
Shielding will be necessary just in some cases, to reduce the EMI, for instance, when there is lot of EM radiation or when we are working with very small signals (guitar pick-ups, mics, line, etc.). In some cases it is required to use shielded cables in interconnects but never for speaker cables.
Well, just using my knowledge, I think shielding will reduce the quality of the audio signal. EM fields generated by cables will be captured inside the shield volume, bouncing back to the cores.
There will be a compromise between having a system immune to EMI and downside effects of a shielded cable. That will depend on the conditions of the audio system. Personally I use balanced unshielded interconnects with excellent results.
NOTE2: Insulation and jacket are more related to the cable's capacitance.
Parallel cables (well separated and not that much), braided cores, Litz wires and other geometries are quite common in the audio business. If we simplify this topic (a lot) we can highlight some geometries:
Paralel cables will have relatively low capacitance and relatively high inductance. If you increase the distance between cores, capacitance and inductance will go even lower and higher respectively (until a limit, of course).
Twisted pairs will increase capacitance and reduce inductance, compared with parallel cables. This is because there will be more material in which store electric charge and because the twiststed cores there will be less induced current.
Multi twisted pairs, also known are braided cables, consist of many twisted pairs in which each pair has positive and negative conductor. This will drastically increase the capacitance and reduce the inductance. This is the same idea that twisted pairs but repeated many times. I built one of these cables following this instructions and I had bad results. Some amplifiers seems to be sensitive to high capacitive loads so the results can be even worst.
Litz wires have many insulated cores transporting the same signal. This claims to reduce the skin effect by reducing the diameter of each core but keeping a relatively high section area (reducing resistance). I haven't tried them but it seems a good idea.
I've seen other geometries but just online. For example this one which is quite exotic. I wanted to build one of these but but I couldn't get the right coaxial cable locally. This is another interesting geometry in which 2 cores are twisted over a Teflon tube. It makes sense as the cores are separated enough to reduce capacitance but because the twists, inductance should go low as well.
NOTE3: Geometry of the cable mainly affects inductance and capacitance.
This is a topic that I don't like very much. You can find a good cable for $100 a meter but then you need 2 connectors, $100 each to match the good cable! If a cable just make signal worst, using a connector will do the same thing.
Every time that the signal has to change from one media to another there will be signal reflections. By changing media I mean changing material, for instance, for connecting the amp with a speaker we would have: copper in the circuit board; the amplifier's connector and it's coat (generally gold or rhodium), the connector in the cable with its coat as well, soldering wire and then the cable. At the other we would have something similar and finally the speaker.
As you can see there is not a smooth path between 2 modules. Lots of materials, geometries, thicknesses and dielectrics around the conductors "stress" the electric signal. In the same way that "the best cable is not a cable" we can say "the best connector is not a connector".
In many places I've read that cables for digital signal do make a difference in sound quality. I think this is partially correct. A very low quality digital cable will create loss of bits and therefore the audio quality will be degraded. This kind of degradation might be quite obvious considering the nature of digital information.
If clock is transmitted together with the audio signal, then a digital cable will make a difference because, even tho the clock is digital, its transmission is interpreted as analogue signal and therefore susceptible to all the "analogue issues". This is called synchronous digital data transfer and it can be found in SPDIF and other audio communication protocols.
Disregarding the example above, cables that are able to transport digital data without bit losses will sound exactly the same. Anybody that understand in depth the nature of digital information should agree with me. USB and Ethernet connections, for instance, will not be affected by the quality of the cables unless the cables are in extremely bad condition or cables don't ………… with the minimum requirements of bandwidth for the specified protocol.
I consider that companies selling super-expensive digital cables are ripping people off. A reasonable $30 ethernet or USB cable is enough and you will not notice any audible difference if you get a $1000 USB cable.
This is the last thing I'll say about digital cables. There will be people saying that they can see/listen the difference in image/sound when using a top quality HDMI cable, for example. In this case it might be true because HDMI transports lots of data and therefore the cable has to match the bandwidth of the audio/video signal. If you want to see a 4K movie (not sure if they exist at this time) you will need a very good HDMI cable because there is a lot of digital data being transmitted. However if you calculate the maximum bandwidth that you need for the best-quality movie that you have and you match that value with an HDMI cable (let's say $200) there will be no differences if you buy a $2000 HDMI cable.
I bought a pair of 3.5 mts 12AWG AntiCables speaker cables for around $150. I connected them instead of a 1mt OFC zip cables 17AWG that I bought in a local shop. To be honest I couldn't hear any difference, but because I just got these new cables I will keep them connected for a few months. I have to say that I'm very happy with my system but speakers cables didn't make any perceptible difference. Now I bought 3.5 mts OFC zip cable 12AWG from the same local shop and I did the test again to match the length of both cables. Any difference? Not for me at least.
Interconnects ae a different story, I built them from scratch: 45cm balanced cables made of 24AWG 999 silver cores, cotton insulation and cores are separated by cardboard disks, so there is basically air between them. I'm quite impress with the sound. Sound staging is very nice, very notorious with acoustic music and voices. The other thing is that I'm not using any connector. The silver cores go directly to the DAC connector in one side and the amp connectors in the other side. In this way I'm saving lots of money ($400 easily) and reducing the changes in the audio signal. I have only compared them with cheap unbalanced cables and the difference is massive. It was not a fare comparison but I don't have other interconnects to compare with.
The other cables that I have in my system are standard USB2 and Ethernet cables to connect my NAS to the Raspberry Pi and then to the DAC. I'm just using standard digital cables, nothing fancy. The only thing that I did was to put a braided nylon sleeve to cover them. They look very cool :-)
There is no "one size fits all" cable. Some systems will work very well with one set of cables and other systems won't. I think that's the main reason why there are so many different opinions about similar products. However this still being a hot topic that creates lots of controversy.
Before buying any cable you should study as much as you can about your system. After that you should check a few cables within your budget. Then you'll have a better chance to get a cable that will match your audio system. Unfortunately it's not always possible to do so. Sometimes the only option is to buy and try different cables until you find the right one. Forums are also useful but they are a double blade sword as you'll find lots of contradictions.
My recommendation is to go for a cable with balanced values for capacitance and inductance. Resistance is generally low enough to forget about it. Good materials are important but don't go crazy. OFC, OOC copper and 999 silver should be fine unless you have a $50.000 system.
Another good options is to share cables with local people so many of you can try different cables without buying them. I have a good friend which is also audiophile and every since in a while we get together and we share our modules and cables.